1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a power transmission belt having a rubber body with a canvas layer adhered thereto to reinforce, and thereby avoid splitting of, the rubber.
2. Background Art
V-ribbed power transmission belts are used in a wide range of industries, including the automotive industry. The design of V-ribbed belts in the automotive industry has been dictated by a number of different requirements and objectives. The V-ribbed belts are required to operate in a compact engine compartment. It is desirable that V-ribbed belts have a substantial life in this environment. At the same time, cost is an important consideration in the design of V-ribbed belts for the automotive industry.
It is known to construct V-ribbed belts with a body having an inside, on which ribs are formed, and an outside, to which a canvas layer is adhered. The canvas layer resists vertical splitting between the inside and outside of the belt, particularly between ribs where the body is the thinnest and thus weakest, and thus becomes significant in determining the overall quality of the belt.
It is known to tenter a canvas layer before application thereof to the belt body. Typically, a plain weave canvas is used which has warp and weft yarns having central axes that are initially crossed at 90.degree.. The tentered canvas produces an angle of 120.degree. between the warp and weft yarns in a lengthwise direction of the belt, as identified by .theta. in FIG. 2 herein.
The above tentered canvas is made from a plain weave canvas made up of 100% cotton fiber, with the thread count of both the warp and weft yarns being 10/10 mm. Each single yarn has a tensile strength of at least 9 N/yarn (N (Newton), is a tensile strength unit, with 1 kgf=9.8N and 1N=0.102 kgf). After tentering, the thread count of both the warp and weft is at least 14/10 mm.
The tentering process requires processing equipment that has a substantial cost. Further, tentering requires an additional manufacturing step, thereby increasing time and expense associated with the manufacture of V-ribbed belts with which the canvas is associated.
The advantage of tentering is simply demonstrated. A plain weave canvas made of spun yarn made of 100% cotton fiber was applied to a V-ribbed belt without tentenrng. The canvas had a yarn count of at least 10/10 mm, with each single yarn having a tensile strength of approximately 9N/yarn. Foreign matter, such as rubber viscous material, stones, and the like, were planted in the grooves between adjacent ribs. Inadequate resistance to vertical splitting of the belt was noticed.
The widthwise static tensile strength of the above canvas layer, after tentering, was approximately 650N/30 mm. The tensile strength of the above plain weave canvas layer, woven under exactly the same conditions without tentering, was 380N/30 mm.
For an untentered cotton fiber plain weave canvas layer to have the same widthwise tensile strength as the tentered canvas of the same composition, in the absence of tentering, the thread count in the belt must be increased to 1.7 times or the single yarn tensile strength must be increased to 1.7 times.
Increasing the thread count by a factor of 1.7 reduces bending resistance for the belt and increases the cost of the canvas and thus the belt made therewith. Increase in the single yarn tensile strength results in an increase in the diameter of the single yarn, which decreases the bending resistance and increases the thickness of the belt in which the canvas layer is incorporated. Increasing belt size is contrary to the aforementioned objective of minimizing belt size as for use in automobile engine compartments.